1. Field of the invention
The invention relates to a system for operating a portable lamp in spaces at risk of an explosion, particularly underground. The invention is directed also to a portable lamp, particularly a miner's cap lamp, and a charging device for charging the rechargeable battery carried with the lamp.
2) Prior Art
Cap lamps suitable for use underground in coalmines are generally operated with incandescent bulbs. The rechargeable battery or battery arrangement supplies the bulb, in use, with a voltage which is safe at the currents which flow through the bulb.
It is known that low pressure gas discharge bulbs, referred to hereinafter briefly as fluorescent bulbs, have a substantially higher light output than incandescent bulbs. They also have a service life many times higher than incandescent bulbs. In this respect, it would be desirable to use fluorescent bulbs instead of the conventional incandescent bulbs in cap lamps. This desirable feature has previously failed due to the problems connected with the switching on of fluorescent bulbs.
Fluorescent bulbs can
a) be started cold by the application of a high starting voltage or PA1 b) be (softly) started after preheating of the cathodes with a substantially lower starting voltage.
Switching on without preheating (cold start) substantially reduces the service life of fluorescent bulbs. The average service life of a cold-started fluorescent bulb is only about one-eighth of a preheated bulb. The preheating of the cathode is however not possible in areas at risk of an explosion because destruction of the bulb or tube during the heating phase can ignite gases and/or dust and initiate gas explosions.